filter circuit and semiconductor device

Abstract

A filter circuit includes a low-pass filter and a calibration circuit calibrating a frequency characteristic of the low-pass filter. The calibration circuit includes a negative feedback circuit and a control circuit.

1. A filter circuit comprising:
 a low-pass filter; and 
 a calibration circuit calibrating a frequency characteristic of the low-pass filter, wherein 
 the calibration circuit includes:
 a feedback circuit which forms an oscillation circuit with the low-pass filter when the filter circuit is in a calibration mode, the oscillation circuit being formed by a negative feedback loop having a gain larger than one; and 
 a control circuit which controls the frequency characteristic of the low-pass filter so as to make an oscillation frequency of the oscillation circuit falls within a predetermined range, when the filter circuit is in the calibration mode, in which 
 
 the feedback circuit comprises:
 a comparator generating a digital output signal from an output signal of the low-pass filter; 
 a buffer generating a feedback signal from the digital output signal of the comparator; and 
 a switch circuit which selects an input signal to output the input signal as an input signal of the low-pass filter when the filter circuit is in a normal mode, and selects the feedback signal to output the feedback signal as the input signal of the low-pass filter when the filter circuit is in the calibration mode, and 
 
 the control circuit comprises:
 a counter which performs a count operation during a predetermined period which is set based on a period of the digital output signal of the comparator, when the filter circuit is in the calibration mode; and 
 an adjustment circuit which compares a count value of the counter and a reference value to adjust the frequency characteristic of the low-pass filter according to a result of the comparison, when the filter circuit is in the calibration mode. 
 
 
     
     
       2. The filter circuit according to  claim 1 , wherein
 the oscillation circuit oscillates independently of a Q-value of the low-pass filter. 
 
     
     
       3. The filter circuit according to  claim 1 , wherein
 a Q-value of the low-pass filter is determined by a ratio of values of filter components including resistors, capacitors and inductors. 
 
     
     
       4. The filter circuit according to  claim 1 , wherein:
 the low-pass filter is a fourth-order Butterworth low-pass filter; and 
 an oscillation frequency of the oscillation circuit is equal to a cut-off frequency of the low-pass filter. 
 
     
     
       5. The filter circuit according to  claim 1 , wherein:
 the low-pass filter is a low-pass filter included in a complex band-pass filter; and 
 a frequency characteristic of the complex band-pass filter and the frequency characteristic of the low-pass filter are determined by a ratio of values of filter components including resistors, capacitors and inductors; and 
 calibration of the frequency characteristic of the complex band-pass filter is realized by the calibration of the frequency characteristic of the low-pass filter. 
 
     
     
       6. A semiconductor device comprising a filter circuit, the filter circuit comprising:
 a low-pass filter; and 
 a calibration circuit calibrating a frequency characteristic of the low-pass filter, wherein 
 the calibration circuit comprises:
 a feedback circuit which forms an oscillation circuit with the low-pass filter when the filter circuit is in a calibration mode, the oscillation circuit being formed by a negative feedback loop having a gain larger than one; and 
 a control circuit which controls the frequency characteristic of the low-pass filter so as to make an oscillation frequency of the oscillation circuit falls within a predetermined range, when the filter circuit is in the calibration mode, in which 
 
 the feedback circuit comprises:
 a comparator generating a digital output signal from an output signal of the low-pass filter; 
 a buffer generating a feedback signal from the digital output signal of the comparator; and 
 a switch circuit which selects an input signal to output the input signal as an input signal of the low-pass filter when the filter circuit is in a normal mode, and selects the feedback signal to output the feedback signal as the input signal of the low-pass filter when the filter circuit is in the calibration mode, and 
 
 the control circuit comprises:
 a counter which performs a count operation during a predetermined period which is set based on a period of the digital output signal of the comparator, when the filter circuit is in the calibration mode; and 
 an adjustment circuit which compares a count value of the counter and a reference value to adjust the frequency characteristic of the low-pass filter according to a result of the comparison, when the filter circuit is in the calibration mode.